It is known that aircraft are furnished with a customary automatic pilot system, comprising a manual control module of said system, this module usually being of the FCU (Flight Control Unit) type. Such a module comprises an adjustment device which allows the adjustment of a certain number of parameters, in particular the altitude of the aircraft, its speed (or its Mach number), its heading (or its flight path), its vertical speed (or its angle of descent), etc.
It is also known that such an adjustment device usually comprises, for the adjustment of a parameter:                at least one selector which is capable of being actuated by an operator;        at least one coder which is driven in rotation by said selector, when the latter is actuated, and which then emits a signal representative of that rotation; and        at least one arithmetic unit which receives the signal emitted by the coder, formats it, makes a computation to deduce the selected value therefrom and is capable of transmitting the selected value thus computed. This selected value is made available to the items of equipment that use it (such as the automatic pilot system in particular), for example by means of an “ARINC 429” type link. It is also communicated, for example, by means of a serial link inside the FCU module, to a display module situated on the front face of the latter.        
In exceptional cases, corresponding for example to an operation of the FCU module in environmental conditions of extreme interference from an electromagnetic point of view or in cases of malfunction, even though extremely brief, of the coder or of a component of one of the portions of the adjustment device of said FCU module, the risk cannot be totally excluded that the value of one of the parameters, selected by a pilot, is modified without there having been an action of said pilot on the selector with the intention of modifying it. Such behavior could be disorientating for the pilot, since the flight of the aircraft would then no longer correspond to the parameters selected by said pilot.